The use of true variable speed compressors has historically been limited to extremely large and expensive commercial air conditioning and refrigeration systems wherein a centrifugal compressor is employed. While variable speed compressors have previously been employed in large air conditioning systems, the use of variable speed reciprocating compressors in smaller systems, such as in heat pumps for residential heating and cooling applications, has not been accepted until recently due to the relative cost of such compressors and their ancillary control equipment and the historical availability of inexpensive power. In general, the efficiencies offered by the use of a true variable speed compressor, in which compressor speed and therefore system capacity is precisely controlled over a continuous range in accordance with the actual demand on the system, have not been sufficiently attractive to offset initial cost considerations in the residential market where profit margins are not so high and wherein the consumer is more easily swayed by the initial cost of a system.
Many less than entirely successful attempts have been made to obtain the benefits associated with the employment of a true variable speed compressor in residential and light commercial air conditioning applications, without actually doing so, by providing for multiple compressor installations which offer discrete capacity steps or by the provision of multi-speed compressors which likewise offer stepped heating or cooling capacities. The response of such systems to the loads they experience, being stepwise rather than continuous, is extremely coarse and inefficient as compared to the response available when a variable speed compressor is employed.
Conditions are changing such that the employment of a true variable speed compressor in certain residential air conditioning applications is becoming more attractive, particularly as consumers become more educated and interested in purchasing energy efficient products. This is especially true of relatively high price products, such as central air conditioning systems, which inherently use large quantities of energy.
Although the use, per se, of a variable speed compressor in air conditioning applications does not present overwhelming technical difficulties, the use of such compressors in small capacity residential systems does present some unique problems which do not exist or which are easily resolved in larger air conditioning systems. Among these is the problem of heat generation by the electronic and electrical components by which the speed of the compressor is controlled.
Residential air conditioners and heat pumps are generally comprised of indoor and outdoor sections. The outdoor section typically houses a compressor, a fan, a heat exchanger, various refrigerant piping/plumbing connections as well as the system controls. All of these components are housed in a relatively small cabinet which sits adjacent the residence or structure and which is exposed, year-round, to outdoor temperature and weather conditions.
Due to the need to protect the control components from the elements, they are generally found in a sealed or entirely closed-in portion of the outdoor section cabinet. The lack of ventilation of this control space exacerbates the problem of cooling the controls therein, particularly when ambient temperatures exceed 100.degree. F. which is a not uncommon condition. Further, if it is decided to cool the controls in such a space with air, the air to be used will almost of necessity be outdoor ambient air. In such cases, elaborate provision must be made to account for the moisture, pollutants, salt and the like which is typically found in the atmosphere and the interaction of which with electronic controls tends to cause deterioration or failure of the controls. It will therefore be appreciated that the use of components such as an inverter to control the speed of a compressor in a residential or light commercial air conditioning application presents some unique and significant difficulties from the standpoint of cooling system controls in a space subject to a large heat load.
Electrical and electronic heat producing components are previously known to have been cooled by immersion in or heat transfer with a liquid coolant. U.S. Pat. Nos. 4,029,141 and 4,144,577 both teach somewhat generic and dedicated cooling structures through which a fluid coolant is pumped to cool particular types of heat producing power components. U.S. Pat. No. 2,643,282, on the other hand, teaches a refrigeration circuit dedicated to the evaporative cooling, by immersion, of electrical components which are otherwise unrelated to the refrigeration system by which such cooling is accomplished.
U.S. Pat. No. 4,084,406 teaches a large centrifugal chiller in which a portion of the refrigerant flowing within the chiller is diverted from the main refrigerant flow path into a branch refrigerant line and into a sealed box containing silicon controlled rectifiers (SCR's). The box fills with liquid refrigerant, immersing the SCR's, until an overflow condition is reached at which time the excess refrigerant flows out of the box and is directed back to a refrigerant sump.
Neither the quantity of refrigerant employed nor cost considerations nor the physical make-up of a residential air conditioning system allow for the direct immersion of system control components in refrigerant for cooling purposes. Nor do space considerations and/or refrigerant amounts in such systems allow for the employment of discrete or independent subsystems dedicated to the cooling of system control components. Therefore, the need exists for both a viable method and apparatus by which the heat generated in compressor speed control components in light commercial/rooftop or residential central air conditioning systems is removed from the space in which the components are located before the employment of a variable speed compressor in these smaller systems becomes practical.